The invention relates to a component placement machine comprising
a machine frame over which printed circuit boards can be transported and on which the printed circuit boards can be fixed, which machine frame is provided with a guide member over which a slide can be moved,
a placement head for placing components on the printed circuit boards, which placement head is coupled to the slide,
a linear motor for driving the slide along the guide member in a drive direction, which motor has a stationary part and a movable part, which movable part is connected to the slide.
Such a component placement machine is known from WO-A1-97/38567. The principle of such a machine is diagrammatically represented in FIGS. 5 and 6, which show, respectively, a plan view and a side view of the machine. The stationary part 7 of the linear motor 5, i.e. a magnetic chuck, is rigidly attached to the machine frame 1. During operation, the coils, which constitute the movable part 6 of the linear motor and are situated on the slide, are energized in order to drive the slide 2 along the guide member 3. As a result, a reactive force is exerted on the magnetic chuck, which force is transmitted to the machine frame. These forces are largest during starting and stopping of the slide. The machine frame starts vibrating and hence also the printed circuit board 8 which is fixed onto the machine frame. The size of the frequency of the vibration and of the amplitude of the vibration are governed to a substantial degree by the size of the reactive forces and the rigidities and masses of the various machine parts. The machine parts serve as a mass-spring system. When the vibrations come close to the natural frequency of such a mass-spring system, unacceptable vibrations having large amplitudes may develop in the machine frame. The position where a component 9 must be placed on a printed circuit board by means of the placement head 4, which is attached on the slide 2, is determined by means of a vision system (not shown). However, said vision system cannot take into account the vibrations of the printed circuit board. The placement of a component takes place immediately after the slide has stopped, i.e. within, for example, approximately 10 xcexcm. Since such vibrations generally do not damp out within this time interval, nor damp at least to an amplitude of a few xcexcm, an inaccuracy occurs during the placement of the component. Postponing the placement until the vibrations have decreased to an acceptable level is unacceptable because it leads to an unacceptable increase of the process time.
It is an object of the invention to minimize the influence of said reactive forces, caused by the drive of the slide, on the machine frame.
To achieve this, the invention is characterized in that the stationary part of the motor is attached onto a force frame which is dynamically disconnected with respect to the machine frame. The reactive forces are now predominantly dealt with in the force frame instead of in the machine frame as in the state of the art. This is important because the position of the slide is measured with respect to the machine frame, not with respect to the force frame. By virtue of the dynamic disconnection, the vibrations caused thereby in the force frame are no longer, or only to a very limited degree, transmitted to the machine frame. Particularly the vibrations with a frequency close to the natural frequency of the whole of machine parts serving as the mass-spring system are not, or hardly, transmitted to the machine frame. As a result, the drive of the slide has no, or hardly any, influence on the position of the printed circuit board. This results in an increase of the accuracy with which the components are placed on the printed circuit board.
The disconnection between the frames could be achieved, for example, by placing both frames separately on the shop floor so as to be disconnected. In this case, the vibrations in the force frame could still be transmitted to the machine frame via the floor. However, the vibrations are then damped to such an extent that the influence thereof on the machine frame is negligible. In practice, however, such a disconnection is almost unfeasible owing to, inter alia, the handling of the machine, the tolerances and calibrations.
Preferably, the force frame is dynamically disconnected with respect to the machine frame, only in the drive direction of the slide. In other directions, the force frame may be coupled to the machine frame. The reason for this being that the reaction forces in a linear motor extend substantially in the direction of the drive of the slide. Thus, the vibrations of the force frame are oriented substantially in this drive direction. As a result, it is possible to mechanically connect the force frame to the machine frame, so that one coherent framework is obtained.
An embodiment thereof is characterized in that the force frame is coupled to the machine frame by means of leaf springs, in such a manner that the leaf springs exhibit a small rigidity in the drive direction of the slide and a great rigidity in the other directions. A leaf spring has a very small mass and, in one direction, a very small rigidity.
Another embodiment is characterized in that a second slide is present on which the placement head is attached and which can be driven along a guide member of the first slide in a second drive direction at right angles to the first drive direction of the first slide by means of a second linear motor, a movable part of which is connected to the second slide, and a stationary part of which is dynamically disconnected with respect to the first slide. This enables the placement head to be positioned in two mutually perpendicular directions with respect to the machine frame and hence with respect to the printed circuit board, the reactive forces of the motors causing no, or hardly any, vibrations in said directions. Preferably, the stationary part is dynamically disconnected with respect to the first slide, only in the drive direction of the second slide. This can be achieved, in the same manner as in the first force frame, by coupling the stationary part of the second linear motor to the first slide by means of leaf springs, in such a manner that the stationary part has a small rigidity with respect to the first slide in the drive direction of the second slide, and a large rigidity in the other directions.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.